1. Field of the Invention
The present invention relates to the characterization of transient behavior of digital circuits.
2. Discussion of the Background Art
Characterizing the transient behavior of digital circuits, i.e. the transition from a logical zero to a logical one and vice versa, has become increasing important for designing as well as manufacturing such digital circuits.
A standard process is to visualize the digital transient behavior by means of oscilloscopes. Actual transient signals are sampled and displayed. FIG. 1 shows a typical example of a visualization provided by an oscilloscope, wherein a plurality of individual transitions 10A and 10B between a logical ‘LOW’ and a logical ‘HIGH’ and a plurality of individual transitions 20A and 20B between ‘HIGH’ and ‘LOW’ are superimposed and thus displayed simultaneously. The representation of FIG. 1 is also called ‘eye diagram’ and is generated by triggering the oscilloscope every period of the data pattern. So all transitions in the pattern are shown simultaneously on the screen.
A further characterization of digital circuits requires determining the so-called Bit Error Rate (BER), i.e. the ratio of erroneous digital signals (Bits) to the total number of regarded digital signals. Typical Bit Error Rates that should not be exceeded are in the range of 10−9 to 10−12, or in other words, one error in 109 to 1012 transmitted bits can be accepted depending on application. That, on the other hand, means that at least three times (109 to 1012) Bits have to be tested error free in order to receive meaningful test results (e.g. >95% confidence level). This, however, leads to long measuring times, so that the characterization of BER generally is a very time-consuming task.
FIG. 2 shows the so-called BER eye diagram as received for the same test as in FIG. 1 but provided by an Agilent® 81200 Data Generator/Analyzer Platform with and Agilent® E4874A Characterization Software Components, both by the applicant Agilent Technologies. The BER eye diagram as a two-dimensional graphical representation is generated using a sweep over delay and threshold of an analyzer. The BER information is displayed by a color coding at each sampled point. The BER can only be small within a portion of the period (because the eye opening is smaller than 100%) and within the right thresholds. The result is an eye pattern with a BER dependent of the sampling point. The result value of BER is determined for each sampling point.
While the eye diagram of FIG. 1 (by the oscilloscope) gives additional information about the pulse form (overshoot etc.), the BER eye diagram of Fig. (by the Agilent 81200) gives an information which bit error rate can be expected depending on the position of the sampling point within the eye.
The actual transient behavior of digital circuits becomes increasingly worse with increasing data transmission rates. Circuits showing sharp (HIGH-to-LOW or LOW-to-HIGH) transitions at low frequencies exhibit ‘long slopes’ for higher frequencies, whereby the actual course of the slope is also subject to jitter or other influences. It goes without saying that with such ‘long and fuzzy slopes’ also the likelihood of (bit) errors increases.
In particular for testing applications in manufacturing environments, it has been shown that the oscilloscope approach (of FIG. 1) is only applicable in so far highly trained personnel is available that can ‘interpret’ such graphic eye diagrams or specific mask matching algorithms are used. BER measurements as shown in FIG. 2, on the other hand, are generally very time-consuming. On the other hand, BER measurements generally consider every data bit, while oscilloscope measurements can only detect a small portion of the data information due to limited sampling rates.